Motors are widely used in the fields such as information processing, aerospace, industrial and agricultural production, and the like. In the starting models of motors, the Variable Frequency Drive (VFD) is a motor controlling device having the highest technical content, the most controlling functions, and the best controlling effect, which adjusts the rotation speed and the torque of the motor by changing its output frequency and voltage. However, if it is required that the load of the motor allows continuous operation at a fixed frequency of the supply network (for example, the multi-pumps or multi-fans function in HVAC application), a contactor (contactors) is/are usually used to bypass for minimizing the power loss. Using the bypass, the motor disconnects from the VFD and connects directly to the supply network after starting.
FIG. 1 shows the structure diagram of a connecting device for motor and supply network of the prior art, wherein Grid represents the supply network, VFD represents the Variable Frequency Drive, M represents the motor, the first contactor S1 represents an output contactor, and the second contactor S2 represents a bypass contactor. In such a connecting device, because of the uncontrollable phase difference and amplitude difference between the voltage of the supply network and the voltage of the motor owing to the switch dead zones of the output contactor S1 and the bypass contactor S2, a switching current surge will occur when the motor is bypassed to the supply network (namely, the motor disconnects from the VFD and connects directly to the supply network).